State of Antarctica: red or blue?

January 21st, 2009 by eric

2) Our results do not necessarily contradict the generally-accepted interpretation of recent East Antarctic cooling put forth by David Thompson (Colorado State) and Susan Solomon (NOAA Aeronomy Lab). In an important paper in Science, they presented evidence that this cooling trend is linked to an increasing trend in the strength of the circumpolar westerlies, and that this can be traced to changes in the stratosphere, mostly due to photochemical ozone losses. Substantial ozone losses did not occur until the late 1970s, and it is only after this period that significant cooling begins in East Antarctica.

3) Our paper — by itself — does not address whether Antarctica’s recent warming is part of a longer term trend. There is separate evidence from ice cores that Antarctica has been warming for most of the 20th century, but this is complicated by the strong influence of El Niño events in West Antarctica. In our own published work to date (Schneider and Steig, PNAS), we find that the 1940s [edit for clarity: the 1935-1945 decade] were the warmest decade of the 20th century in West Antarctica, due to an exceptionally large warming of the tropical Pacific at that time.

So what do our results show? Essentially, that the big picture of Antarctic climate change in the latter part of the 20th century has been largely overlooked. It is well known that it has been warming on the Antarctic Peninsula, probably for the last 100 years (measurements begin at the sub-Antarctic Island of Orcadas in 1901 and show a nearly monotonic warming trend). And yes, East Antarctica cooled over the 1980s and 1990s (though not, in our results, at a statistically significant rate). But West Antarctica, which no one really has paid much attention to (as far as temperature changes are concerned), has been warming rapidly for at least the last 50 years.

Why West Antarctica is warming is just beginning to be explored, but in our paper we argue that it basically has to do enhanced meridional flow — there is more warm air reaching West Antarctica from farther north (that is, from warmer, lower latitudes). In the parlance of statistical climatology, the “zonal wave 3 pattern” has increased (see Raphael, GRL 2004). Something that goes along with this change in atmospheric circulation is reduced sea ice in the region (while sea ice in Antarctica has been increasing on average, there have been significant declines off the West Antarctic coast for the last 25 years, and probably longer). And in fact this is self reinforcing (less sea ice, warmer water, rising air, lower pressure, enhanced storminess).

The obvious question, of course, is whether those changes in circulation are themselves simply “natural variability” or whether they are forced — that is, resulting from changes in greenhouse gases. There will no doubt be a flurry of papers that follow ours, to address that very question. A recent paper in Nature Geosciences by Gillet et al. examined trends in temperatures in the both Antarctic and the Arctic, and concluded that “temperature changes in both … regions can be attributed to human activity.” Unfortunately our results weren’t available in time to be made use of in that paper. But we suspect it will be straightforward to do an update of that work that does incorporate our results, and we look forward to seeing that happen.